The use of fluoropolymer membranes, sheets, or films as ion-transporting barriers in various processes is becoming more and more common. Of particular interest is the field of electrochemistry wherein fluoropolymer membranes are employed as permselective ion-transport barriers, whereby a liquid medium is substantially barred from passage, yet certain ions contained in the liquid are transported through the barrier.
Various fluoropolymers contain functional pendent acid groups which contain sulfur, phosphorus, or carbon. Salts or other derivatives of these acid groups are known.
For instance, various fluoropolymer membranes having functional acid groups, e.g., sulfonic acid groups, phosphonic acid groups and/or carboxylic acid groups have been suggested for use in separating anolyte from catholyte in electrolytic chlor-alkali cells. Derivatives of the functional sulfonic acid, phosphonic acid and/or carboxylic acid groups on the fluoropolymer chains, which may enhance or moderate the ion-transport properties are, for example, R.sub.f COOM, R.sub.f COOX, R.sub.f SO.sub.3 X, R.sub.f SO.sub.3 M, R.sub.f PO.sub.3 X.sub.2, R.sub.f PO.sub.3 M.sub.2, and the like, where X is Na.sup.+, Li.sup.+, K.sup.+, or NH.sub.4.sup.+, M is an amine radical which may be substituted or unsubstituted, and R.sub.f represents the fluoropolymer segment to which the functional group is attached.
The fluoropolymers are generally prepared by polymerizing fluorine-substituted hydrocarbons having carbon-to-carbon unsaturation, such as tetrafluoroethylene, hexafluoropropylene, vinylidene fluoride, and the like, with the functional sulfonic acid groups and/or carboxylic acid groups attached thereto.
It is known that polytetrafluoroethylene (PTFE) can be particulated and then thermally degraded to monomeric tetrafluoroethylene (TFE) and the monomer may then be re-used.
Fluoropolymers which have become popular in the electrochemical field, as membrane material, include those which have been offered under the tradename of NAFION by E. I. DuPont De Nemours & Company. Methods for making fluoropolymer membranes are shown, e.g., in U.S. Pat. Nos. 3,282,875; 3,624,053; 3,849,243; 3,718,627; 3,969,285; 4,178,218; and 4,259,226.
U.S. Pat. No. 4,259,226 discloses certain fluoropolymers which are soluble in various solvents.
Patents which disclose preparation of fluoropolymers containing pendant carboxylic acid groups are, e.g., U.S. Nos. 3,506,635; 4,151,053; 4,255,240; and 4,176,215 and British Pat. No. 1,145,445.
Patents which disclose particulate forms of acid ion exchange resins for use in Bisphenol processes are, e.g., U.S. Nos. 4,107,218 and 4,191,843.
Over a period of time the fluoropolymer membranes can become damaged and/or become contaminated with deleterious deposits, or otherwise have reduced effectiveness, to an extent such that their replacement is required. There are economic and other practical reasons for salvaging these relatively expensive polymers by putting them to further use.
It is an object of the present invention to avoid having to discard used fluoropolymer membranes.
A further object is to provide a means for altering used fluoropolymer membranes before further use of them.
A still further object is to provide a means for substantially removing unwanted ingredients from a used fluoropolymer resin.